In 2246, he was living on Tarsus IV when four thousand people were executed on the orders of GovernorKodos. Nine eyewitnesses, including Leighton, could identify the governor on sight. Leighton and Kirk appeared to have been personal acquaintances, perhaps as a result of their shared experiences on Tarsus IV. He was tormented by what happened that day for the next twenty years.

In 2266, he diverted the USS Enterprise to Planet Q, ostensibly to confirm his discovery of a new synthetic food that could totally end the threat of famine on Cygnia Minor, a nearby colony. In fact, he diverted the Enterprise because he believed Anton Karidian, a traveling performer, was actually Kodos. He hoped Kirk, another of the eyewitnesses, would help him in his quest to bring Kodos to justice. Kirk wasn't convinced until, ironically, Leighton was found dead. Lenore Karidian, insanely desperate to protect her father from his past, had murdered Leighton; he was the seventh victim of her quest to eliminate all those who could identify Kodos. He was survived by his wife Martha. (TOS: "The Conscience of the King")

Although it was not explicitly stated, conversational context strongly suggested that the left side of Leighton's face was ruined during the massacre at Tarsus IV.